Actually it is 7
When TNA were doing weekly PPV @ the Asylum, they ran the 1st "Anniversary" show in the Summer of 2003 (because TNA started in June 2002), and then another one in the Summer of 2004 (which included the very 1st King of the Moutain match)
So when they moved to the Impact Zone in 2004, during the post-FSN (internet/Real Player) days of TNA in 2005, they started calling the anniversary show "Slammiversary", because they started doing monthly PPVs
2003(1), 2004(2), 2005(3), 2006(4), 2007(5), 2008(6), and 2009(7)
Btw, if anyone wants to watch the 1st ... AND VERY 1st TNA ANNIVERSARY SHOW
http://www.megavideo.com/?v=HNEPCQAD
enjoy, it was a good show... you'll enjoy it

... a lot of good matches and surprises, I cannot believe it's been 7 years since this aired